1. Field of the Invention
The present invention relates to a braking system for use in an automotive vehicle and particularly to a hydraulic braking system comprising a tandem master cylinder and a hydraulic booster which outputs hydraulic power pressure supplied from a power source in response to the depression of a brake pedal.
2. Description of the Prior Art
In a conventional braking system for an automotive vehicle, there are provided a plurality of hydraulic circuits connecting wheel brake cylinders with a hydraulic braking pressure generator such as a master cylinder, so that when one of the hydraulic circuits is broken, braking operation is achieved by the rest of the hydraulic circuits. In general, a tandem master cylinder is used in a conventional dual circuit system.
In order to reduce the force required to operate a brake pedal during a braking operation, the hydraulic braking system is provided with a servo unit which is commonly referred to as a servo or a booster and which utilizes compressed air, intake manifold vacuum (for a vacuum booster), or hydraulic pressure (for a hydraulic booster) as a power source. The hydraulic booster is a booster which actuates the hydraulic braking pressure generator such a the master cylinder by the hydraulic power pressure supplied from the power source in response to depression of the brake pedal. For example, the Japanese Patent Laid-open Publication No. 59-209948 has disclosed a system associating the hydraulic booster with the tandem master cylinder which operates as an ordinary tandem master cylinder when the hydraulic booster is not operated.
With employment of such hydraulic booster, it has been proposed to use the hydraulic booster, in the hydraulic braking system, as a dynamic hydraulic pressure generator in addition to the master cylinder. In other words, a hydraulic pressure boosted by the booster (hereinafter referred to as boost pressure) in response to the depression of the brake pedal is applied directly to a hydraulic circuit. For example, as shown in Japanese Patent Laid-open Publication No. 59-227552, boost pressure of the hydraulic booster is applied to rear wheel brake cylinders in a front-and-rear dual circuits system in order to reduce the stroke of the brake pedal. However, if the hydraulic power pressure is lost due to a failure of the operation of the power source, the braking force of the rear wheels disappears, although the braking force of the front wheels can be maintained by the master cylinder.
It is also known from Japanese Laid-open Publication No. 62-155167 that the boost pressure of a hydraulic booster can be transmitted to a pressure chamber of the tandem master cylinder for pressuring certain wheel cylinders. According to such a system, the various effects such as the shortening of the stroke of the brake pedal and so on are obtained. In case of the loss of hydraulic power pressure, the system functions as an ordinary tandem master cylinder and the braking force of all of the wheels can be maintained. In that system, a pressurizing reservoir (e.g., see brake chamber 60 in the publication) having a large capacity is provided, so that enough brake fluid is available when the hydraulic power pressure is not supplied to the second fluid chamber (e.g., see chamber 28 in the publication) of the tandem master cylinder due to the loss of operation of the power source. However, since the second fluid chamber of the tandem master cylinder is communicated with the boost chamber (e.g., see booster operation chamber 88 in the publication) of the hydraulic booster via the pressurizing reservoir, a closed space is formed therein when the communication with the power source is interrupted or the operation of the power source is ceased. Accordingly, since the volume of the boost chamber is expanded by the operation of the hydraulic booster in the above case, the volume of the closed space expands and a negative pressure is generated in that space. This negative pressure results in a decrease of the output hydraulic pressure of the master cylinder and a corresponding loss of the braking force. The occurrence of this phenomenon is especially evident when the brake pedal is depressed many times in a short period. As mentioned above, the pressurizing reservoir is recognized as being effective to compensate for the loss of operation of the power source and so on. However, it is difficult to form such reservoir in the housing.